The expectation is that future GW observations will give us some information on the nature and physics of the near-horizon region of black holes (BHs) and if these regions exhibit any unexpected structure. Alternatives to BHs, that is, objects without event horizons, were recently constructed, known as exotic compact objects (ECOs) [1]}-[2]}. The existence of any structure at near-horizon scales would generate a series of “echoes” of the primary gravitational wave signal, produced during the ringdown phase [3]}, [4]}. The LIGO data have already been analyzed on the presence of echoes [5]}, [6]}, [7]}.
